Despite the proliferation of broadband Internet technology in the last few years, many regions of the world, particularly rural and low-income areas, still do not have access to broadband services. Huge costs involved are the main obstacle to creating new infrastructure in such areas for existing broadband wired or wireless technologies like digital subscriber line (DSL), cable and satellite. But if broadband could be served through electric power lines, there would be no need to build a new infrastructure for last-mile access. So wherever electricity is available, there could be broadband.
In this perspective, broadband over power lines (BPL) technology seems to emerge as a new business prospective for established telecom operators, utility companies and system manufacturers to provide high-speed broadband services that can cover each and every home or office due to almost omnipresence of power lines. With no need for new wiring or major infrastructure deployment, BPL creates an alternative broadband solution that could lead to lower prices for broadband consumers. Thus it is hoped that BPL has the potential to become an effective means for last-mile delivery of broadband services and may offer a competitive alternative to other high-speed Internet access technologies.
BPL building blocks
BPL is a power-line communication technology that allows high-speed digital data transmission over utility power lines. By utilising the combination of technological principles of radio, power engineering, electromagnetic compatibility, networking and modem technology, it offers instantaneous access to high-speed Internet from in-house electrical outlet. In order to access broadband services, subscribers need to install a modem that plugs into an ordinary wall outlet.
Fig. 1: BPL building blocks
BPL systems use the existing electrical power lines as a transmission medium to provide high-speed telecommunications capabilities by coupling radio frequency (RF) energy onto the power line. In order to carry high-speed data, BPL uses radio spectrum ranging from high-frequency (3-30 MHz) to lower portion of very high-frequency (30-300 MHz) allocated to other over-the-air communication services. Because the electric current (50 Hz) and RF (typically 2-80 MHz for BPL) have different frequencies, the two don’t interfere with each other.
There are two predominant types of BPL communication configurations: Access BPL systems that couple RF energy onto medium-voltage (MV) and low-voltage (LV) power lines; and in-home BPL networks, which use existing electrical outlets available within a home or office for the provision of a local-area network (LAN). A typical BPL network set-up is shown in Fig. 1.
The existing three-tiered power grid hierarchy, which comprises HV, MV and LV transmission lines, is exploited to carry the high-speed digital data over a broad range of frequencies without causing any significant interference to the rightful incumbent users of those frequency bands. The electricity is generated from power plants (thermal, hydro, nuclear, wind turbines or solar), which is synchronised three-phase, offset by 120-degree, AC power of the order of thousand volts at the line rate of 50 cycles per second. Three phases are chosen to get nearly peak value at any given instant, which results in a good compromise between cost and performance. More phases could be used but this implies more cost with only a slight improvement in performance.
The AC power generated is now ready for its journey to the customers. As mentioned earlier, a three-tiered hierarchy is used to transmit this AC power to distant end-users. Power transferred over lines is given by the product of voltage and carrying current.
For a given line resistance, which depends on the line material and line length, the power loss is given by the product of line resistance and square of the carrying current. So in order to reduce power loss in the lines and transfer maximum generated power to long-distant end-users, current must be made as small as possible and voltage as large as possible. That’s why transmission substations located next to power plants use large transformers to step up generator output voltage, thus allowing megawatts of power transmission over distances of 400 km or more.
At power substations, voltages are stepped down and lines branched out to cover larger areas. This is performed successively, transforming and branching out from extremely high voltage (typically 345 to 785 kV) to HV (typically 115 to 230 kV), and then from HV to MV (typically 2.4 to 69 kV), and finally from MV to LV (typically 120V to 600V) for delivery to homes or offices.
HV lines transmit power over distances that are tens of kilometres, but are very noisy and not used to carry broadband signals. Such a high voltage contains infinite number of harmonic components, and if it spikes at the frequency of the RF used to transmit data, it will cancel out that signal and the data transmission will be dropped or damaged en-route.
MV lines are capable of transmitting power over a few kilometres, while LV lines cover only a few hundred metres. MV and LV lines are preferred for use in BPL data transmission because of their low noise level.
In order to use MV/LV lines for broadband services, BPL systems are designed to operate in the frequency spectrum range of 2 to 80 MHz. BPL signal may be injected onto power lines between two phase conductors, between a phase conductor and the neutral conductor, or on a single phase or neutral conductor. The BPL signals are injected into and extracted from MV/LV lines through inductive or capacitive couplers.
Injector is a device that aggregates user data onto the power lines and provides an interface between power line and BPL operation support system that is connected to the Internet backbone. Concentrator provides an interface between power lines carrying the BPL signal and the households.
In inductive coupling, BPL signal couples onto the line by wrapping inductors around the line. On the other hand, capacitive coupling uses a capacitor for coupling and the signal is modulated onto the line voltage. Inductive couplers are known to be rather lossy, but since they require no physical connection to the network, they are safer to install on energised lines than capacitive couplers.
Since HF signals have the rare ability to support long-distance point-to-point communications due to line noise, signal attenuation and limitations on the amount of signal power that can be injected onto power lines without causing unreasonable interference for other spectrum users, repeaters are required in between the transmission and reception ends. This can be done by using MV couplers to couple the broadband signal off the MV line so that it can be regenerated, if necessary, and amplified before being fed back onto the MV line through another coupler.
The distribution transformers that change voltage levels between MV and LV lines are particularly harsh on the weak broadband signal. These transformers, due to their high inductance, act as low-pass filters and appear as open circuits for the passage of high-frequency BPL signals. This implies that BPL signals going between MV and LV lines need to bypass the transformers. Typically, a bypass box can be used, which may consist of coupler and built-in repeating functionality.
Connectivity to the backhaul network (Internet, PSTN, UMTS, 2G, 3G, 4G, CDMA, WiMAX, etc) is provided through the operation support system (OSS) coupled to an MV distribution line. The OSS converts data formats, aggregates and concentrates uplink data streams, provides routing functionality, helps allocate bandwidth and resources, generates billing and charging data, and provides various backhaul Ethernet interfaces to fibre-optic or wireless connections. It consists of various servers like authentication and authorisation server, dynamic host control protocol server, domain name system server and billing server to perform the required back-end tasks.
BPL can be deployed either as end-to-end BPL or as hybrid BPL. The end-to-end BPL system uses both the access BPL and the in-house BPL, i.e., power lines are used all the way from the power substation to the end user. In this case, BPL signal can either bypass the MV/LV transformer or go through the transformer (Fig. 1).
In hybrid BPL, the bypass box does not couple the broadband signal to/from the LV line but converts it to/from a wireless format and delivers it to the wireless access point (AP) located on the pole (Fig. 1).
Fig. 2: BPL electromagnetic model
For end-to-end BPL, bypass boxes and LV couplers must be installed on all LV lines, and in-house BPL modems are required. For hybrid BPL, bypass boxes with wireless conversion boards, wireless APs and existing standard wireless user modems are required.
Architecture and standards
Based on seven-layer open system interconnection model defined in ISO-7498, BPL systems are designed to work on a two-layer architecture comprising physical (PHY) and medium access control (MAC) layers.
The PHY layer defines electrical and physical specifications for devices, i.e., relationship between a device and a physical medium. It includes all the electrical power line distribution system and the in-home electrical wiring down to the wall sockets. The major functions of PHY layer include establishment and termination of a connection to a communication medium, communication flow control, modulation and coding.
MAC layer, consisting of MAC lower sub-layer and logical link control layer, provides an interface between PHY layer and higher layers for connection to physical media.
At initial stage, manufacturers of BPL systems have developed their own proprietary solutions for injectors, repeaters, extractors and couplers. These solutions have been implemented in a variety of system architectures, which feature different modulation techniques and designs at PHY, MAC and LLC layers. Such systems have raised incompatibility issues.
In order to combat channel impairments (noise, multipath, strong channel selectivity, non-linear channel characteristics), a number of different technologies at PHY layer have been employed that range from spread spectrum to orthogonal frequency-division multiplexing (OFDM). OFDM is a type of frequency-division multiplexing system that provides better channel throughput, better spectrum efficiency and robustness against frequency selectivity because all of the underlying sub-carriers are orthogonal to one another.
There are numerous modulation and access design techniques that can be used in association with OFDM. These include binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 16-quadrature amplitude modulation (QAM) and 6-bit, 64-constellation QAM. Various forward error correction codes are also used.
Since BPL access networks operate in a shared transmission medium where subscribers compete to use the same transmission resources, MAC has been designed for point-to-multipoint applications and is based on collision-sense multiple-access with collision avoidance (CSMA/CA). MAC layer also describes how secure communications are delivered, by using secure key exchange during authentication and encryption (using advanced encryption standard or data encryption standard) during data transfer.
In order to streamline the functionality of BPL systems, Open Power line communication European Research Alliance, European Telecommunications Standards Institute, Universal Powerline Association, European Committee for Electrotechnical Standardization, HomePlug Powerline Alliance and Institute of Electrical and Electronics Engineers (IEEE) have developed standards for BPL systems.
The IEEE has constituted IEEE P1675 ‘Standard for Broadband over Power Line Hardware,’ IEEE P1775 ‘Powerline Communication Equipment—Electromagnetic Compatibility (EMC) Requirements—Testing and Measurement Methods’ and IEEE P1901 ‘Draft Standard for Broadband over Power Line Networks: Medium Access Control and Physical Layer Specifications.’
HomePlug Powerline Alliance has developed in-house BPL specifications: HomePlug 1.0 for speeds up to 14 Mbps and HomePlug AV for speeds greater than 100 Mbps.
Interference issues. Most BPL systems are designed to operate in the frequency spectrum from 2 to 30 MHz, but occasionally up to 80 MHz, using MV and LV power distribution network lines. The frequency spectrum from 2 to 30 MHz constitutes a limited natural resource that includes the HF band (3 to 30 MHz), which is being used for many decades by shortwave radio stations, military, aviation agencies, etc. Above the HF band, the frequency spectrum from 30 to 54 MHz is reserved for use by public service and business communications. The spectrum from 54 to 80 MHz hosts television channels (channel numbers 2 to 5) with a small segment used for some other emergency services.
However, because BPL uses some of the radio frequencies used for over-the-air radio systems, mutual interference is a major problem. In order to mitigate interference at some specific frequency, OFDM may be used as it has the ability to notch these specific frequencies.
Moreover, power lines are unshielded and act as radiating antennae for the signals they carry, resulting in interference with communication systems of broadcast radio, military, aviation agencies, etc.
Electromagnetic fields associated with access-BPL system are shown in Fig. 2 and can be categorised as guided mode, coupler fields and radiation mode. Guided mode serves to transport signal energy along the line. Signal energy decays rapidly in directions perpendicular to the line but slowly along its length. Coupler fields are associated with the coupler itself rather than the power line. Radiation fields are considered to be a source of interference because these fields decay relatively slowly.
Interference associated with BPL can be broadly categorised into two groups: near-field (0.62(D3/λ)1/2<r<2D2/λ) radiation and far-field (r>2D2/λ) radiation, where ‘r’ is the distance from the radiator, ‘D’ is the largest linear dimension of the radiator and ‘λ’ is signal wavelength. Near-field effects persist along the entire length of the wire and may cause serious interference issues. Far-field effects can be quite troublesome because ionospheric propagation of HF radiation can travel thousands of kilometres. In order to reduce these radiations, a balanced configuration of two MV wires driven differentially and spaced appropriately can be employed.
Power line noise. MV/LV power lines are inherently very noisy due to changing nature of the load connected to power lines, number and types of line branches, length of line branches, type of power line equipment connected (such as capacitor banks and transformers), and impedance mismatch caused by unterminated stubs and line branches. In addition, on/off switching of capacitor banks used to correct power factor and switching power supplies often introduce noisy harmonics into the line.
Unlike twisted-pair (used in DSL) or shielded coaxial cable, power lines have no inherent noise cancellation mechanism. In order to cope up with noisy environment of power lines, OFDM along with a number of channel interference noise ratio based modulations such as QAM, BPSK and QPSK may be a solution. OFDM is very robust against frequency selectivity but any time-varying characteristic of the channel limits system performance. Time variations deteriorate the orthogonality of the sub-carriers, resulting in inter-carrier interference. To eliminate inter-carrier interference, a guard time is inserted with a length longer than the duration of the impulse response of the channel. The insertion of guard time has the penalty of a loss in the signal-to-noise ratio that further increases the bandwidth requirement.
Bandwidth issues. Each distribution transformer feeds power to 10 to 50 houses/offices. Since bandwidth to the transformer is limited and the same power line is shared by all these houses, there is scarcity of bandwidth available to each customer. Moreover, BPL is a contention-based system (CSMA/CA), which again imposes additional challenges. Similar to DSL, speeds (bit-rates) of BPL systems available to customers are also dependent on the distance between the supplying substation and the customer’s home.
Security issues. BPL signal propagates in a LAN-like manner, which makes detection and interception of neighbouring transmissions simple. So in order to prevent interception of legitimate customers’ data by unauthorised intruders, strong authorisation, authentication and data encryption algorithms such as DES and AES should be used.
BPL in a nutshell
BPL technology is a union of two applications in a single system, holding great promise as a ubiquitous broadband solution that would offer a viable alternative to cable, digital subscriber line, fibre and wireless broadband solutions. Additionally, it offers the ability to employ intelligent power line networks that make use of supervisory control and data acquisition devices. As standardisation, interference mitigation and improvement in technology are on their way, the future of BPL looks very bright.
The author is a junior telecom officer at Bharat Sanchar Nigam Limited, currently working in Ludhiana, Punjab. He holds Ph.D in electronics engineering from Indian Institute of Technology, BHU, Varanasi, and has authored and co-authored several research papers in peer-reviewed national/international journals including IEEE and conference proceedings. His current research interests include wired and wireless technologies for high-speed Internet access; use of renewable energy sources; and analysis, design and simulation of high-power high-frequency microwave devices and systems for communication purposes